1. Field of the Invention
The present invention relates to a liquid crystal display and a driving method thereof.
2. Description of Related Art
Liquid crystal displays (“LCDs”) are widely used as flat panel displays and include two panels provided with two kinds of field-generating electrodes (e.g., a plurality of pixel electrodes and one large planar reference electrode) on their inner surfaces and a liquid crystal layer interposed between the panels. A difference between the voltages applied to the field-generating electrodes generates an electric field in the liquid crystal layer and the orientations of liquid crystal molecules in the liquid crystal layer change depending on the intensity of the electric field. The changing of the orientations of the liquid crystal molecules alters the polarization of the light passing through the liquid crystal layer, which results in the variation of light transmittance by polarizers attached on outer surfaces of the panels. Therefore, the light transmittance can be controlled by adjusting the voltage difference between the field-generating electrodes to change the intensity of the electric field.
LCDs include a plurality of pixels arranged in a matrix and a plurality of display signal lines for transferring signals to the pixels. Each pixel includes a liquid crystal capacitor and a switching element connected to the liquid crystal capacitor and the display signal lines. The liquid crystal capacitor includes two terminals and a liquid crystal dielectric between the two terminals. The two terminals of the liquid crystal capacitor are formed by the pixel electrode and a portion of the reference electrode opposite the pixel electrode. An example of the switching element is a thin film transistor (“TFT”) having a control terminal and input and output terminals.
The display signal lines include a plurality of gate lines transmitting gate signals for turning on/off the switching elements and a plurality of data lines transmitting data signals to be applied to the pixel electrodes of the liquid crystal capacitors. In detail, each switching element is connected to one of the gate lines and one of the data lines such that the switching element is turned on upon receipt of a gate-on voltage of the gate signal to transfer the data signals from the data line to the liquid crystal capacitor and the switching element is turned off upon receipt of a gate-off voltage of the gate signal not to transfer the data signals. The reference electrode receives a predetermined voltage called a “reference voltage.”
The direction of the electric field is repeatedly reversed to prevent the electric and physical characteristics of the liquid crystal layer from deteriorating due to the long-term application of a unidirectional field. To reverse the direction of the electric field, the polarity of the data voltages applied to the pixel electrode with respect to the reference voltage applied to the reference electrode is periodically reversed.
However, such polarity inversion brings an undesirable phenomenon called “flicker” that is a blinking of an image-displaying screen. The flicker is resulted from the reduction of the voltage of the reference electrode due to a kickback voltage originated from the switching characteristics of the switching element. The voltage drop of the reference electrode due to the kickback voltage is proportional to a magnitude of the kickback voltage.
The magnitude of the kickback voltage depends upon the position of the kickback voltage on the panels, which shows drastic position dependency especially in a row direction, i.e., an extension direction of the gate lines. A difference between the gate-on voltage and the gate-off voltage, which determines the magnitude of the kickback voltage, varies along the gate lines due to the delay of the gate signal. In detail, the kickback voltage has the largest value at the position at which the gate signal is applied, and it becomes smaller as it goes along the gate line because the signal delay of the gate signal becomes larger.
One of the techniques for solving this problem is to apply a plurality of reference voltages with different magnitudes to at least two points on the reference electrode.
For example, the magnitude difference of the kickback voltage along the gate line is compensated by applying different reference voltages to two points of the reference electrode, e.g., two opposite ends of the reference electrode in the row direction.
The technique assumes that the voltage drop of the kickback voltage shows linearity due to the delay of the gate signal. However, since the actual kickback voltage shows non-linearity, the technique is not effective.
Moreover, conventional techniques generally use a variable resistor for adjusting the reference voltages applied to the reference electrode. The adjustment of the resistance of the variable resistor may affect a potential difference between the two opposite end points of the reference electrode, thereby making the flicker tuning difficult.